linux-kernel - Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic transfers

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Date:	Fri, 07 Aug 2015 13:23:20 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:	Peter Ujfalusi <peter.ujfalusi@...com>,
	Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	nsekhar@...com, linux-omap@...r.kernel.org,
	linux-serial@...r.kernel.org, john.ogness@...utronix.de,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic transfers

On 08/07/2015 12:39 PM, Russell King - ARM Linux wrote:
> On Fri, Aug 07, 2015 at 05:44:03PM +0200, Sebastian Andrzej Siewior wrote:
>> On 08/07/2015 05:29 PM, Russell King - ARM Linux wrote:
>>> On Fri, Aug 07, 2015 at 11:08:48AM -0400, Peter Hurley wrote:
>>>> [ + Greg KH ]
>>>>
>>>> On 08/07/2015 09:57 AM, Russell King - ARM Linux wrote:
>>>>> As it is something that the driver has _not_ supported, you are clearly
>>>>> adding a feature to an existing driver.  It's not a bug fix.
>>>>>
>>>>>>> If something else has been converted to pause channels and that is causing
>>>>>>> a problem, then _that_ conversion is where the bug lies, not the lack of
>>>>>>> support in the omap-dma.
>>>>
>>>> FWIW, the actual bug is the api that silently does nothing.
>>>
>>> Incorrect.
>>>
>>> static int omap_dma_pause(struct dma_chan *chan)
>>> {
>>>         struct omap_chan *c = to_omap_dma_chan(chan);
>>>
>>>         /* Pause/Resume only allowed with cyclic mode */
>>>         if (!c->cyclic)
>>>                 return -EINVAL;
>>>
>>> Asking for the channel to be paused will return an error code indicating
>>> that the request failed.  That will be propagated back through to the
>>> return code of dmaengine_pause().
>>>
>>> If we look at what 8250-dma.c is doing:
>>>
>>>                 if (dma->rx_running) {
>>>                         dmaengine_pause(dma->rxchan);
>>>
>>> It's 8250-dma.c which is silently _ignoring_ the return code, failing
>>> to check that the operation it requested worked.  Maybe this should be
>>> WARN_ON(dmaengine_pause(dma->rxchan)) or at least it should print a
>>> message?
>>
>> I think this is what Peter meant by saying "silently does nothing".
> 
> Maybe Peter should phrase his replies better.  "the actual bug is the
> api that silently does nothing." to me means he is complaining that
> dmaengine_pause() had no effect.  _That_ is what I'm objecting to,
> and claiming that Peter's comment is wrong.

Yes, I missed that the api included a return code which the 8250 dma
code should be checking.

> He's now blaming me for snide remarks.  I could call his one above an
> incorrect snide remark against the quality of DMA engine code.

You'd be reading a lot into my statement.

> He's
> totally wrong, and been proven wrong by my analysis above, plain and
> simple.
> 
> He should now accept that he's wrong

Done.

> and move along with sorting out
> this mess _without_ requiring optional features to be implemented in
> other subsystems to fix bugs in code he's supposed to be maintaining.

This is simply a bug that flew under the radar, much like every other
bug that gets fixed daily in mainline.

The omap-serial driver which doesn't use dma is still the preferred
stable driver for omap, for the moment.

One of the main features of the 8250_omap integration was the addition
of dma support. Without it, 8250_omap is ttyO in ttyS clothing.

Regards,
Peter Hurley
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